Future Collectible Handguns
First, remember that condition is king in gun collecting. Here are my speculations:
Colt revolvers: No longer made, a rare snake when they were. All are escalating at this time, but especially the Pythons and Diamondbacks.
Blued and pinned S&W revolvers: These are also no longer made, and even the modern reproductions are far beneath the older guns in quality. The nickel brethern are collectible too.
Both of these are already collected, and are thus not the sleepers asked for though........In the sleeper category, I believe these will prove fruitful:
Ruger MKII auto pistols: These are great fun, accurate, reliable, and represent a nostalgic time for many people. They are the Ruger auto pistol zenith, and no longer made. Expect these to escalate just like the K-22 Outdoorsman did. Condition will be everything. High Standard auto pistols will be a close second, and perhaps the MKI as well.
Glocks: People collect weird stuff, and Glocks do have a following. I expect the first generation Glocks to become collectible. They are significant guns, and they remain shootable.
Sistema Colt 1911s: The supply has dried up, it's a genuine GI 1911, long ignored just as the M1911A1 was when it was plentiful.
S&W Victory Models: I expect these to reach M1911A1 prices, especially the US Property marked ones.
All imported C&R handguns: I expect there to finally be restrictions on importing handguns that effectively kills the C&R trade. Expect Walther P1s, Makarovs, Tokarevs, and a host of other interesting, good quality military surplus weapons to skyrocket.
The coming changes in our government's leadership will potentially bring about changes for gun owners. Many are already ordering Bushmasters as a hedge against another assault weapon ban. Some of the other trends I believe, are more predictable.